There are well-known methods for studying or monitoring the evolution of an oil-bearing deposit for example, which comprise positioning of several sensors at determined intervals along the well or wellbore and the coupling thereof with the wall of the wellbore, triggering of a seismic source at the surface or in the same well as that in which the sensors are located or another one, and transmission to a central surface recording station of the signals picked up in response to the emitted waves, so as to locate by means of conventional processings the position of the underground reflectors. Significant results are obtained by carrying out spatial filtering of the picked up signals. Several sensors arranged in one or several sondes are displaced successively in a large number of different locations along the well and the recordings of the signals picked up are combined as is well-known by specialists.
Achieving such seismic operations generally requires that the well or wells containing the production equipments be temporarily shut down which is very disadvantageous and that the production equipments be removed to install the monitoring sondes and anchor them against the casings thereof.
U.S. Pat. Nos. 4,775,009 and 4,986,350 granted to the same applicant relate to a technique for avoiding unavailability of a well, which mainly consists of installing a large number of sensors outside casings for casing one or several wells and of connecting these sensors through transmission lines to a surface control and recording station. The casing being set, the sensors located in the annular space around it are coupled with the formations surrounding the well by the injected cement. The sensors are generally arranged in reception units fixed to the outside of the casing, fitted in housings placed on the outer wall of some of the sections thereof or arranged in intermediate connections specially provided. These reception units comprise housings for geophones and electronic modules allowing the signals received by the geophones to be adapted before being transmitted to a surface station by cables going up to the surface. This technique is suited when the number of reception units is limited because each one of them must be separately connected to a surface installation.
When a well is equipped for oil production, it is also well-known to place sensors in the annular space between the casing and the tubing, as described for example in patent FR-2,656,034 granted to the same applicant, and to press them against the casing so as to obtain proper acoustic coupling with the formations surrounding the well.
Patent application FR-91/15,483 filed by the same applicant describes a process for connecting together various reception units, notably seismic sondes, which mainly consists of arranging between them sealed hydraulic pipes resistant to the pressure prevailing in the well, so as to provide a common space isolated from the outside environment and to pass electrical conductors inside these pipes.
When one or several wells are to be equipped with sensors located behind a casing for example and intended to be definitively embedded in cement, sensors of very various types, likely to gather a large number of different types of data are to be set, so as to analyze and to follow better the evolution of the penetrated zone. As will be specified in the description hereafter, in connection with FIG. 1 notably, it is necessary for seismic applications with a seismic source located at the surface or in another well for example, to place sensors such as geophones, allowing seismic waves or the components thereof to be received along several axes, in a plurality of different depth levels, within a frequency range limited at most to 1000 or 1500 Hz. For applications relating to the passive monitoring of the evolution of a production zone, it is necessary to have several triaxial sensors placed in the well towards the top of the deposit or reservoir and capable of receiving signals within a frequency band that is wider than for the previous applications. One tries thus to pick up the seismic or acoustic activity that originates in the deposit during the bringing in thereof. The passband useful for this monitoring is generally wider than in the previous case. Other sensors may be placed in the well, in the part thereof penetrating the deposit to carry out tomography with a seismic source arranged in another well. It is also often useful to arrange in the well, at the level of the production zones, sensors which will be referred to hereafter as condition sensors, to measure various thermodynamic parameters: pressure, temperature, or parameters such as flow rates, measured by various instruments, etc.
The mode of transmission, to a surface installation, of the signals delivered by each of these types of sensors may be varied. For passive monitoring operations, the sensors must be connected permanently or almost permanently to the surface installation so that the significant signals picked up, which are often of random nature, may be transmitted at any time. Connection to the surface installation of the sensors used for achieving induced seismic recording cycles is only necessary during active seismic prospecting runs, and during time intervals of some seconds at each cycle generally, when permanent monitoring has to be stopped.
Connection of condition sensors located in the production zone to measure parameters such as temperatures or pressures may also be permanent or almost permanent. However, the inquiry frequency thereof may be much lower since the evolution of the parameters measured is generally slow.
The transmission system installed must put up with various data transfer modes. Transfer, to the surface installation, of the data obtained during active or passive seismic operations is generally achieved in a coded digitized form. In passive monitoring phases for example, the rate must be sufficient to transmit in real time the signals coming from several sensors arranged respectively at various depths, so as to appraise correctly the direction from which an acoustic shaking comes and the emission distance thereof. The rate necessary for the transmission thereof may reach in practice 1 Mbit/s. In induced seismic prospecting applications achieved with a large number of sensors, the necessary rate may greatly exceed this value. For other transmission modes, the required rate is definitely different.
Numerous condition sensors sense the parameters they measure in analog form such as a proportionate intensity current for example, and it is important that they may be transmitted to the surface.
The transmission system to be installed must thus be particularly flexible to process such various data transfers. Since it is stationary and may therefore often not be recoverable, and since it is often used for long-duration prospecting and monitoring operations, it has to be relatively simple, cheap and reliable. The lay-out thereof must also be designed so that a possible local failure does not challenge the reliability as a whole. Besides, especially when there is a large number of different types of devices distributed along the well, it is important that the permanent power consumption of each of them is moderate to remain within power limits that can be withstood by the transmission cables generally used in wells.